1. Field of the Invention
The present invention relates to a driving circuit of a thin-film transistor electroluminescent (TFT-EL) display, and more particularly to a control TFT in a pixel for driving a light-emitting diode (LED) in a TFT-EL display, which reduces current leakage therein.
2. Description of the Related Art
Currently, flat panel Liquid crystal displays (LCDs) are widely implemented in various electronic devices, such as notebooks and televisions.
The predominant type of LCD currently employed is the thin film transistor LCD (TFT LCD), which is operated by active addressing, meaning each pixel is individually addressed by a corresponding thin film transistor. Due to the maturity of TFT fabrication processes, large size LCD devices are also widely available.
Conventional TFT LCDs, however, are limited by poor transmittance. This is particularly true with regard to color flat panel displays. Hence, a backlight module is typically coupled to the TFT LCD to provide adequate light transmittance. A major drawback of backlight modules, however, is excessive power consumption, which limits the application of TFT LCDs in portable electronic devices. Another drawback of backlight modules is size, limiting which LCDs from attaining a thinner profile. For example, when a typical fluorescent tube lamp is utilized in a backlight module coupled to an LCD panel, the thickness of the resulting LCD monitor increases by ¾ to 1 inch accompanied by an increase in weight.
Electroluminescent (TFT-EL) displays have been forwarded as the ideal solution as they offer low power consumption and eliminate the need for backlight modules. Typically, the individual pixels of a TFT-EL display can be self-addressed to emit light and therefore do not require additional backlight to provide excellent transmittance.
The developing electroluminescent (TFT-EL) displays are known as organic electroluminescence device or organic light emitting diode (OLED). Referring to FIG. 1, a circuit for driving an organic light-emitting diode (OLED) in a TFT-EL display is shown. In the circuit, the OLED (O10) is controlled by two thin-film transistors (T10 and T20) and one capacitor (C10). When the scan line (S10) is driven, the switch TFT T10 is switched on and the signal from the data line (D10) is input to the capacitor C10. When the capacitor C10 is charged, a current source V supplies a current to drive the organic light-emitting diode (O10) according to the I-V characteristic of the control TFT (T20) and the voltage level of the capacitor (C10). The control TFT (T20) is also called a “driving TFT”.
The capacitor (C10) is charged when the switch TFT (T10) is switched on and the switch TFT (T10) is switched off when charging is complete. The gate voltage of the control TFT (T20) is controlled by the capacitor (C10) and therefore the current is determined accordingly. The luminance and gray scale of the OLED (O10) is determined by the current from the control TFT (T20). After the switch TFT (T10) is switched off, any leakage of the control TFT (T20) will affect the capacitance of the capacitor (C10) and decrease the current, thereby affecting the luminance and gray scale of the OLED (O10).